Speed sensor instability detection

ABSTRACT

A speed sensor instability detection and correction system for use with a vehicle includes a speed sensor output variation extractor monitoring and recording change in output of a speed sensor over time. A speed sensor output variation comparison module detects speed sensor instability based on a comparison between the speed sensor output variation and a predetermined threshold. An output speed determination module selectively determines output speed in alternative ways based on whether speed sensor instability is detected.

FIELD OF THE INVENTION

The present invention generally relates to acceleration based tractioncontrol systems, and more particularly to a control system and methodfor detecting wheel slip during output speed instability due to speedsensor instability.

BACKGROUND OF THE INVENTION

As a motor vehicle is being driven, it is not uncommon for one of thedriven wheels to lose its grip on the road. To promote safety, tractioncontrol systems have been developed for detecting wheel spin andreducing wheel torque in response to wheel spin. In one traction controlsystem, described in U.S. Pat. No. 6,182,002 B1, vehicle acceleration ismeasured and used to detect wheel spin. Another traction control systemis described in U.S. patent application Ser. No. 10/601,063, filed Jun.20, 2003 (Attorney Docket No. GP-302694), entitled “Wheel Slip Detectionand Torque Management”, which is commonly assigned and herebyincorporated by reference in its entirety. This traction control systemuses pulley speed of a transmission, such as a continuously variabletransmission (CVT), to detect wheel slippage.

In other systems, wheel speeds are monitored to detect wheel spin. Forexample, in vehicles having anti-lock brake systems (ABS), wheel speedsensors are used to provide information to a traction control system. Asa result, wheel slippage can be detected directly.

Speed sensors also play other important roles in a motor vehicle. Forexample, a CVT ratio is determined as a function of throttle pedalsignal, pulley speed signal, and torque converter turbine speed signal.As a result, various output speeds are combined to affect control of thevehicle. Further, some output speeds, such as pulley speed, may playmultiple roles.

A problem therefore arises in each of the forgoing systems and methodswhen a speed sensor exhibits instability resulting, for example, from amissing tooth or electrical instability. For example, when instabilityoccurs in the output pulley speed or turbine speed, the ratio of the CVTis perturbed. Also, when instability occurs in output pulley speed oroutput wheel speed, engine flare may result due to incorrect detectionof wheel slippage.

SUMMARY OF THE INVENTION

In one embodiment in accordance with the present invention, a speedsensor instability detection and correction system for use with avehicle includes a speed sensor output variation extractor that monitorsand records change in output of a speed sensor over time. A speed sensoroutput variation comparison module detects speed sensor instabilitybased on a comparison between the speed sensor output variation and apredetermined threshold. An output speed determination moduleselectively determines output speed in alternative ways based on whetherspeed sensor instability is detected.

In another embodiment in accordance with the present invention, avehicle includes a speed sensor that detects a speed of a vehicle. Aspeed sensor instability detector detects speed sensor instability basedon a change in output of the speed sensor over time. An output speeddetermination module employs a median filter to filter output of thespeed sensor when the speed sensor instability detector indicates speedsensor instability.

In a further embodiment in accordance with the present invention, amethod for detecting wheel slip based on output speed of a vehicleincludes determining whether output speed instability is detected. Themethod also includes generating a median filtered output speed whenoutput speed instability is detected. The method further includesdetermining whether wheel slip is present based on the median filteredoutput speed when output speed instability is detected.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a functional block diagram of a vehicle including a tractioncontrol system according to one embodiment of the present invention;

FIG. 2 is a flow diagram depicting wheel speed slippage detection inaccordance with the present invention;

FIG. 3 is a flow diagram depicting speed sensor instability detection inaccordance with the present invention; and

FIG. 4 is a flow diagram depicting output speed determination inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses. For purposes of clarity, the same referencenumbers will be used in the drawings to identify similar components.

A vehicle having a traction control system in accordance with oneembodiment is referred to generally in FIG. 1 by reference number 10.The vehicle 10 includes an engine 12 having a plurality of cylinders 14,each with one or more intake valves and/or exhaust valves (not shown).The engine 12 is configured with a fuel injection system 16 and anignition system 18. An output of the engine 12 is coupled by a torqueconverter 22 having a torque converter clutch 23, a continuouslyvariable transmission 24, a driveshaft 26 and a differential 28 todriven front wheels 30 a. Rear wheels 30 b are non-driven in the presentembodiment. The transmission 24 is controlled by a controller 20.

One or more wheel speed sensors 32 transmit speed signal(s) to thecontroller 20. In the embodiment shown in FIG. 1, driven wheels 30 a andnon-driven wheels 30 b have speed sensors 32 a and 32 b respectively. Inother embodiments, fewer than four wheel speed sensors may be provided.For example, one sensor is used on one of the non-driven wheels. Shaftspeed sensors 34 transmit input signals to the controller 20.

An electronic throttle controller (ETC) 36 adjusts a throttle 38 in anintake manifold 40 based upon a position of an accelerator pedal 42 anda throttle control algorithm that is executed by the controller 20. Thethrottle 38 adjusts an output torque that drives the wheels 30 a. One ormore sensors (not shown), for example a manifold pressure sensor and amanifold air temperature sensor, may be provided in the intake manifold40.

An accelerator pedal sensor 44 generates a pedal position signal that isoutput to the controller 20 based on a position of an accelerator pedal42. A position of a brake pedal 46 is sensed by a brake pedal sensor 48,which generates a brake pedal position signal that is output to thecontroller 20. Other sensor inputs collectively indicated by referencenumber 50 are used by the controller 20 for traction control. Suchinputs include a signal 52 indicating engine speed, a vehicle speedsignal 54, an accelerator position signal 56 and a throttle positionsignal 58. One such signal to the controller 20, a trans throttle signal60, indicates throttle pressure in the transmission 24. Signals 62 and64 indicate transmission input and output pulley speeds.

The present invention operates in the above-mentioned context to detectoutput speed sensor instability and take appropriate corrective action.It should be readily understood that instability in output pulley speed,output turbine speed, and output wheel speed sensor signals may beutilized in accordance with the present invention.

FIG. 2 illustrates wheel slippage detection in accordance with thepresent invention. Accordingly, when output speed instability isdetected as at 70, a median filter is used to filter the output speed atstep 72. In contrast, when output speed instability is not detected at70, then a Kalman filter is used to filter the output speed at 74. It isenvisioned that an output pulley speed sensor instability is detected at70, and that the output pulley speed sensor signal is filtered in steps72 and 74. Then, actual speed ratio is calculated at step 76 andcontinued detection of output speed instability is enabled at step 78.The actual speed ratio is used at step 80 to calculate wheel axletorque, and this wheel axle torque is used at step 82 with the outputspeed filter value of step 72 or 74 to determine an expected outputspeed derivative. In parallel, the same output speed filter value isused at step 84 to calculate an actual output speed derivative. Then,the actual output speed derivative of step 84 is compared at 86 to theexpected output speed derivative of step 82. Thus, if the expectedoutput speed derivative is greater than the actual output speedderivative, then wheel slippage is undetected at step 88. Otherwise,wheel slippage is detected at step 90.

FIG. 3 illustrates detection of output speed instability based on speedsensor instability. Accordingly, if the sensor is not in a default stateand the instability detection is currently enabled as at 94, theninstability detection is allowed to take place. In order to haveflexibility on different applications, a selectable instability bit, orflag, is added to govern whether detection is enabled. The detection ispreferably performed when the bit is set to true. However, the detectionis disabled when the speed sensor has failed, and any previouslydetected instability is undetected at step 96 if detection is disabledfor any reason.

Speed sensor instability detection according to the present inventionincludes comparing a median sensor speed to a predetermined threshold at98 to ensure that instability can be differentiated from a low speedoutput at low speeds. Thus, if the median output speed is not too low at98, then the absolute value of the speed sensor variation for one loopis compared to a predetermined threshold at 100. If the variation ishigh enough for the loop, then an instability counter is incremented at102. Each loop represents a small amount of time, so this counter allowsloop to loop instability to be tracked over time to improve accuracy.Thus, if the counter reaches a predetermined threshold during apredetermined amount of time as at 104, then instability is deemedreliably detected and the counter is reset to zero at 106. Instabilitythen remains detected until undetected, for example, at step 96.However, instability can also be undetected at step 108 if instabilityhas remained detected for a predetermined amount of time and theinstability counter is zero as at 110. These conditions ensure thatother software processes will have time to appropriately react todetected instability before it is undetected, and also that detectedinstability is not immediately undetected as a result of concurrentlyresetting the instability counter at step 106. Additionally, the counteris reset at step 112 if the counter does not exceed the threshold withinthe predetermined amount of time as at 114. This reset results inundetection of instability on a subsequent loop at 108 if enough timehas passed at 110. It is envisioned that the detection may be employedwith respect to an output pulley speed sensor, a turbine speed sensor, awheel speed sensor, or additional types of sensors detecting speed ofvehicle component or the vehicle itself.

FIG. 4 illustrates alternative procedures for determining output speedbased on output speed instability detection in accordance with thepresent invention. Accordingly, a Kalman filter is employed at step 116to filter the output of the speed sensor if sensor instability is notdetected at 118. If an antilock brake system employing wheel speedsensors is not present at 120, then a median filter is employed tofilter output of the speed sensor at 122. This filter takes a pointsample and rejects the extreme value. However, the present invention isalso capable of recalculating output speed based on a maximum outputspeed of the wheel sensors and a transfer gear of the CVT at step 124.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the current invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, the specification and the following claims.

1. A speed sensor instability detection and correction system for usewith a vehicle, comprising: a speed sensor output variation extractorthat monitors and records change in output of a speed sensor over time,thereby extracting a speed sensor output variation; a speed sensoroutput variation comparison module that detects speed sensor instabilitybased on a comparison between the speed sensor output variation and apredetermined threshold; and an output speed determination module incommunication with said speed output variation comparison module thatselectively determines output speed based on whether speed sensorinstability is detected.
 2. The system of claim 1 wherein said speedvariation comparison module compares speed output variation to thepredetermined threshold over a predetermined amount of time, and detectsspeed sensor instability if the variation exceeds the threshold for thepredetermined amount of time.
 3. The system of claim 2 wherein saidspeed variation comparison module continues to detect speed sensorinstability until the variation no longer exceeds the predeterminedthreshold for a second predetermined amount of time.
 4. The system ofclaim 1 wherein said output speed determination module determines outputspeed as a Kalman filtered output of the speed sensor when speed sensorinstability is not present.
 5. The system of claim 1 wherein said outputspeed determination module determines output speed as a median filteredoutput of the speed sensor when speed sensor instability is present. 6.The system of claim 1 wherein said output speed determination modulecalculates output speed based on a maximum output of the speed sensorand a transfer gear of the vehicle when speed sensor instability ispresent.
 7. A vehicle, comprising: a speed sensor that detects a speedof the vehicle; a speed sensor instability detector that detects speedsensor instability based on a change in output of said speed sensor overtime; and an output speed determination module that employs a medianfilter to filter output of said speed sensor when said speed sensorinstability detector indicates speed sensor instability.
 8. The vehicleof claim 7 wherein said speed sensor instability detector makes acomparison between the change in wheel speed over time and apredetermined threshold, and detects instability based on results of thecomparison over time.
 9. The vehicle of claim 7 further comprising awheel slip detector that calculates speed ratio based on the filteredoutput of said speed sensor, calculates axle torque based on the speedratio, calculates an output derivative based on the median filteredoutput speed, calculates an expected derivative based on the axletorque, and determines whether wheel slip is present by comparing theoutput derivative to the expected derivative.
 10. The vehicle of claim 7wherein said output speed determination module employs a Kalman filterto filter output of said speed sensor when said speed sensor instabilitydetector does not indicate speed sensor instability.
 11. A method fordetecting wheel slippage based on output speed of a wheeled vehicle,comprising: determining whether output speed instability is detected;generating a median filtered output speed when output speed instabilityis detected; and determining whether wheel slippage is present based onthe median filtered output speed when output speed instability isdetected.
 12. The method of claim 11 wherein said determining whetheroutput speed instability is detected includes monitoring change in apulley speed sensor output signal over time.
 13. The method of claim 12wherein said determining whether output speed instability is detectedfurther includes making a comparison between change in pulley speed overtime and a predetermined threshold.
 14. The method of claim 13 whereinsaid determining whether output speed instability is detected furtherincludes detecting instability based on results of the comparison overtime.
 15. The method of claim 11 wherein said generating a medianfiltered output speed further includes filtering an output signal of apulley speed sensor with a median filter.
 16. The method of claim 11wherein said determining whether wheel slippage is present includescalculating speed ratio based on the median filtered output speed. 17.The method of claim 16 wherein said determining whether wheel slippageis present further includes calculating wheel axle torque based on thespeed ratio.
 18. The method of claim 17 wherein said determining whetherwheel slippage is present further includes: calculating an outputderivative based on the median filtered output speed; and calculating anexpected derivative based on the wheel axle torque.
 19. The method ofclaim 18 wherein said determining whether wheel slippage is presentfurther includes comparing the output derivative to the expectedderivative.
 20. The method of claim 11 further comprising: generating aKalman filtered output speed when output speed instability is notdetected; and determining whether wheel slippage is present based on theKalman filtered output speed when output speed instability is notdetected.